The present invention relates to a vane for use in various displacement-type actuators and a method for producing the vane.
The operation of a vane actuator exemplified by a hydraulic vane pump is first explained referring to FIG. 8. In this vane pump which may be used as a hydraulic motor as it is, a rotor 11 is fixed to a shaft 22 and rotatable in a closed space defined by a cam ring 13 and a pair of side blocks (not-shown) fluid-tightly fixed to both side ends of the cam ring 13. A member such as the cam ring 13 which is brought into contact with the rotor 11 may sometimes be called "contact member" herein.
Each vane 1 is substantially in a flat rectangular parallelepiped shape, both wide side surfaces thereof being slidably guided in a radial groove 12 of the rotor 11, and both narrow side surfaces thereof being slidably guided along inner surfaces of the side blocks. The rotor 11 rotates together with the vanes 1 in the direction shown by the arrow 20. During the rotation of the rotor 11, a top surface 1a of each vane 1 is always pressed into contact with a cam surface 14 of the cam ring 13 by a centrifugal force, a spring force, an outward force exerted by a pressurized hydraulic fluid entering into a space between the vane 1 and the radial groove 12 of the rotor 11.
Each pump room 15 defined by the rotor 11, the adjacent vanes 1, the cam surface 14 of the cam ring 13 and the side blocks has a volume variable depending on the rotation of the rotor 11, with the maximum volume at upper left and lower right positions and the minimum volume at lower left and upper right positions in FIG. 8. Accordingly, the hydraulic fluid is sucked into the pump room 15 through intake ports 16, 16 provided in the side block and discharged from the pump room 15 through discharge ports 17, 17 provided in the side block.
The top surface 1a of the vane 1 which is in slidable contact with the cam surface 14 is arcuately or roundly projecting, such that good sealing is always kept between the vane 1 and the cam surface 14 regardless of a relative angle of the vane 1 to the cam surface 14. Therefore, it has conventionally been considered that the arcuately projecting or round top surface 1a of the vane 1 should have high precision in dimension, straightness and surface roughness.
To achieve high precision in dimension and surface roughness, the top surface 1a of the vane 1 has conventionally been ground by a creep feed grinding wheel 9 as shown in FIG. 9, which has a grinding groove 9a formed on a circumferential surface by a dressing tool. The grinding wheel 9 is moved back and forth while rotating along the top surface 1a of the vane 1 in the direction perpendicular to the plane of the paper presenting FIG. 9. Though this grinding method can provide the round top surface 1a of the vane 1 with high precision in straightness and surface smoothness, it is a time-consuming process poor in efficiency, making the total production cost of the vanes high.
In view of these facts, methods of producing vanes without creep feed grinding have been proposed.
Japanese Patent Laid-Open No. 58-206896 discloses a method for producing a vane comprising the steps of subjecting a flat bar having a round surface at a top end to a normalization treatment; cutting the flat bar to a predetermined length to provide a vane; hardening the vane by a heat treatment; grinding surfaces of the vane to predetermined dimensions except for the round top surface to provide a vane having a predetermined cross section; assembling the resultant vanes into a vane pump such that the round top surface of each vane is pressed onto a cam surface of a cam ring; and carrying out a running-in operation of the vane pump to wear away a decarburized layer formed on the round top surface of each vane in the normalization step, such that the top surface of the vane is provided with a shape adapted to the cam surface of the cam ring. However, this method is not usable because it generates a large amount of wear dust which causes various troubles.
Japanese Patent Laid-Open No. 2-308993 discloses a method for producing a vane by plastic working such as drawing or extruding. As shown in FIGS. 11(a), 11(b), both side surfaces 30c, 30c' of a flat bar 30 are rolled by a pair of rolls 21, 22 having circumferential grooves 23, 24. Each of the circumferential grooves 23, 24 has a flat bottom surface 23a, 24a, a pair of rounded corners 23b, 23c, 24b, 24c to impart to the flat bar 30 flat side surfaces 30c, 30c', a rounded top surface 30a and a chamfered bottom end surface 30b. Because of rolling in the thickness direction, however, high precision in shape, straightness and surface smoothness cannot be achieved in the top surface 30a of the flat bar 30. As a result, the resultant vane fails to provide sufficient sealing without finish-grinding of a top surface.
Like the above two methods, vanes have been produced from flat bars which are cut to predetermined length by band saws, grinders, presses (shearing), etc., at a proper production stage. These cutting methods, however, are insufficient in cutting precision in length, surface roughness, scars, straightness, rectangularity, etc. In the case of shearing, the cross-sectional shape of the resultant vane may inevitably be distorted at cut ends, making it necessary to shear-cut the flat bar with a proper margin which is then removed by finish-grinding. Thus, the cutting method and the subsequent finish-grinding are also important factors determining the production cost of the vanes.
To improve the overall efficiency of an actuator, it is important to decrease friction between the vane and the cam ring while suppressing leaks. For this purpose, the vanes and the cam ring should have sufficiently precise dimension with minimum surface roughness. Since an inner surface of the cam ring is usually ground by a small-diameter grinding wheel supported by a projected shaft, the precise grinding of the inner surface of the cam ring cannot be carried out efficiently. The cam ring receives larger grinding pressure in an inner portion than in both opening (edge) portions from the grinding wheel, leading to larger grinding in both edge portions of the cam ring. As a result, a slight taper is inevitably formed in a ground inner surface of the cam ring within a range of about 0.5 mm from each opening of the cam ring.
The vane is usually barrel-finished, leaving a droop in a round top surface of the vane within a range of about 0.1 mm or more from the end thereof. Accordingly, when these vanes are assembled with the cam ring, leaking of a hydraulic fluid inevitably takes place because of the gap between the droops of the vanes at both ends of their round top surfaces and the tapered opening ends of the cam ring in its inner surface.